The X-linked form of severe combined immune deficiency (XSCID) is due to mutations of the gene encoding the gamma chain of the interleukin-2 receptor (IL2-Rgamma). Patients with XSCID have profound T lymphopenia and poor mitogen responsiveness. In female carriers, T lymphocyte progenitors expressing the mutant IL-2Rgamma ene are at selective disadvantage and do not mature into peripheral blood T lymphocytes. Treatment of SCID is bone marrow transplantation (BMT), using either HLA- identical sibling, matched unrelated donor or T-lymphocyte depleted HLA- haploidentical marrow. The use of haploidentical BMT is complicated by the need for pre-transplant conditioning to reliably ensure engraftment, graft-versus-host disease (GVHD), an increased risk of infection, and hypo- or agammaglobulinemia post-BMT. A canine form of XSCID has marked similarities to the human disease, including mutation of IL-2Rgamma, selective disadvantage to T cells expressing the mutant IL-2Rgamma gene in carrier females, and curability by BMT. The present proposal will test the use of in utero transplantation of hematopoietic stem cells (HSC) to treat XSCID. By use of curative conventional BMT, fertile XSCID male and female breeding pairs will be developed so that litters with 100% inheritance of XSCID will be produced. Reagents to identify canine CD34+ CD38- progenitors will be developed to allow purification of clinically useful numbers of canine HSC. The ability of histocompatible CD34+ CD38- cells to achieve hematopoietic and lymphoid chimerism after histocompatible post-natal transplantation will be tested first. Histocompatible, haploidentical, and mismatched transplants of CD34+ CD38- cells will be tested on XSCID fetuses. Outcome measures will include T and B lymphoid engraftment and function, GVHD, and clinical outcome. Potential advantages of in utero stem cell therapy over haploidentical BMT include: pre-transplant chemotherapy would not be necessary; the use of purified HSC will prevent GVHD; T lymphocyte reconstitution would take place in the protected environment of the uterus; and B lymphocyte function may be better. The studies will establish a model therapy directly applicable to a human disease and yield important information regarding the biology and clinical utility of in utero stem cell transplantation.